The present invention relates to a tonometer system for measuring intraocular pressure by accurately providing a predetermined amount of applanation to the cornea and detecting the amount of force required to achieve the predetermined amount of applanation. The system is also capable of measuring intraocular pressure by indenting the cornea using a predetermined force applied using an indenting element and detecting the distance the indenting element moves into the cornea when the predetermined force is applied, the distance being inversely proportional to intraocular pressure. The present invention also relates to a method of using the tonometer system to measure hydrodynamic characteristics of the eye, especially outflow facility.
The tonometer system of the present invention may also be used to measure hemodynamics of the eye, especially ocular blood flow and pressure in the eye's blood vessels. Additionally, the tonometer system of the present invention may be used to increase and measure the eye pressure and evaluate, at the same time, the ocular effects of the increased pressure.
Glaucoma is a leading cause of blindness worldwide and, although it is more common in adults over age 35, it can occur at any age. Glaucoma primarily arises when intraocular pressure increases to values which the eye cannot withstand.
The fluid responsible for pressure in the eye is the aqueous humor. It is a transparent fluid produced by the eye in the ciliary body and collected and drained by a series of channels (trabecular meshwork, Schlemm's canal and venous system). The basic disorder in most glaucoma patients is caused by an obstruction or interference that restricts the flow of aqueous humor out of the eye. Such an obstruction or interference prevents the aqueous humor from leaving the eye at a normal rate. This pathologic condition occurs long before there is a consequent rise in intraocular pressure. This increased resistance to outflow of aqueous humor is the major cause of increased intraocular pressure in glaucoma-stricken patients.
Increased pressure within the eye causes progressive damage to the optic nerve. As optic nerve damage occurs, characteristic defects in the visual field develop, which can lead to blindness if the disease remains undetected and untreated. Because of the insidious nature of glaucoma and the gradual and painless loss of vision associated therewith, glaucoma does not produce symptoms that would motivate an individual to seek help until relatively late in its course when irreversible damage has already occurred. As a result, millions of glaucoma victims are unaware that they have the disease and face eventual blindness. Glaucoma can be detected and evaluated by measuring the eye's fluid pressure using a tonometer and/or by measuring the eye fluid outflow facility. Currently, the most frequently used way of measuring facility of outflow is by doing indentation tonography. According to this technique, the capacity for flow is determined by placing a tonometer upon the eye. The weight of the instrument forces aqueous humor through the filtration system, and the rate at which the pressure in the eye declines with time is related to the ease with which the fluid leaves the eye.
Individuals at risk for glaucoma and individuals who will develop glaucoma generally have a decreased outflow facility. Thus, the measurement of the outflow facility provides information which can help to identify individuals who may develop glaucoma, and consequently will allow early evaluation and institution of therapy before any significant damage occurs.
The measurement of outflow facility is helpful in making therapeutic decisions and in evaluating changes that may occur with time, aging, surgery, or the use of medications to alter intraocular pressure. The determination of outflow facility is also an important research tool for the investigation of matters such as drug effects, the mechanism of action of various treatment modalities, assessment of the adequacy of antiglaucoma therapy, detection of wide diurnal swings in pressure and to study the pathophysiology of glaucoma.
There are several methods and devices available for measuring intraocular pressure, outflow facility, and/or various other glaucoma-related characteristics of the eye. The following patents disclose various examples of such conventional devices and methods:
______________________________________ PATENT NO. PATENTEE ______________________________________ 5,375,595 Sinha et al. 5,295,495 Maddess 5,251,627 Morris 5,217,015 Kaye et al. 5,183,044 Nishio et al. 5,179,953 Kursar 5,148,807 Hsu 5,109,852 Kaye et al. 5,165,409 Coan 5,076,274 Matsumoto 5,005,577 Frenkel 4,951,671 Coan 4,947,849 Takahashi et al. 4,944,303 Katsuragi 4,922,913 Waters, Jr. et al. 4,860,755 Erath 4,771,792 Seale 4,628,938 Lee 4,305,399 Beale 3,724,263 Rose et al. 3,585,849 Grolman 3,545,260 Lichtenstein et al. ______________________________________
Still other examples of conventional devices and/or methods are disclosed in Morey, Contact Lens Tonometer, RCA Technical Notes, No. 602, December 1964; Russell & Bergmanson, Multiple Applications of the NCT: An Assessment of the Instrument's Effect on IOP, Ophthal. Physiol. Opt., Vol. 9, April 1989, pp. 212-214; Moses & Grodzki, The Pneumatonograph: A Laboratory Study, Arch. Ophthalmol., Vol. 97, March 1979, pp. 547-552; and C. C. Collins, Miniature Passive Pressure Transensor for Implanting in the Eye, IEEE Transactions on Bio-medical Engineering, April 1967, pp. 74-83.
In general, eye pressure is measured by depressing or flattening the surface of the eye, and then estimating the amount of force necessary to produce the given flattening or depression. Conventional tonometry techniques using the principle of applanation may provide accurate measurements of intraocular pressure, but are subject to many errors in the way they are currently being performed. In addition, the present devices either require professional assistance for their use or are too complicated, expensive or inaccurate for individuals to use at home. As a result, individuals must visit an eye care professional in order to check their eye pressure. The frequent self-checking of intraocular pressure is useful not only for monitoring therapy and self-checking for patients with glaucoma, but also for the early detection of rises in pressure in individuals without glaucoma and for whom the elevated pressure was not detected during their office visit.
Pathogens that cause severe eye infection and visual impairment such as herpes and adenovirus as well as the virus that causes AIDS can be found on the surface of the eye and in the tear film. These microorganisms can be transmitted from one patient to another through the tonometer tip or probe. Probe covers have been designed in order to prevent transmission of diseases but are not widely used because they are not practical and provide less accurate measurements. Tonometers which prevent the transmission of diseases, such as the "air-puff" type of tonometer also have been designed but they are expensive and provide less accurate measurements. Any conventional direct contact tonometers can potentially transmit a variety of systemic and ocular diseases.
The two main techniques for the measurement of intraocular pressure require a force that flattens or a force that indents the eye, called "applanation" and "indentation" tonometry respectively.
Applanation tonometry is based on the Imbert-Fick principle. This principle states that for an ideal dry, thin walled sphere, the pressure inside the sphere equals the force necessary to flatten its surface divided by the area of flattening. P=F/A (where P=pressure, F=force, A=area). In applanation tonometry, the cornea is flattened, and by measuring the applanating force and knowing the area flattened, the intraocular pressure is determined.
By contrast, according to indentation tonometry (Schiotz), a known weight (or force) is applied against the cornea and the intraocular pressure is estimated by measuring the linear displacement which results during deformation or indentation of the cornea. The linear displacement caused by the force is indicative of intraocular pressure. In particular, for standard forces and standard dimensions of the indenting device, there are known tables which correlate the linear displacement and intraocular pressure.
Conventional measurement techniques using applanation and indentation are subject to many errors. The most frequently used technique in the clinical setting is contact applanation using Goldman tonometers. The main sources of errors associated with this method include the addition of extraneous pressure on the cornea by the examiner, squeezing of the eyelids or excessive widening of the lid fissure by the patient due to the discomfort caused by the tonometer probe resting upon the eye, and inadequate or excessive amount of dye (fluorescein). In addition, the conventional techniques depend upon operator skill and require that the operator subjectively determine alignment, angle and amount of depression. Thus, variability and inconsistency associated with less valid measurements are problems encountered using the conventional methods and devices.
Another conventional technique involves air-puff tonometers wherein a puff of compressed air of a known volume and pressure is applied against the surface of the eye, while sensors detect the time necessary to achieve a predetermined amount of deformation in the eye's surface caused by application of the air puff. Such a device is described, for example, in U.S. Pat. No. 3,545,260 to Lichtenstein et al. Although the non-contact (air-puff) tonometer does not use dye and does not present problems such as extraneous pressure on the eye by the examiner or the transmission of diseases, there are other problems associated therewith. Such devices, for example, are expensive, require a supply of compressed gas, are considered cumbersome to operate, are difficult to maintain in proper alignment and depend on the skill and technique of the operator. In addition, the individual tested generally complains of pain associated with the air discharged toward the eye, and due to that discomfort many individuals are hesitant to undergo further measurement with this type of device. The primary advantage of the non-contact tonometer is its ability to measure pressure without transmitting diseases, but they are not accepted in general as providing accurate measurements and are primarily useful for large-scale glaucoma screening programs.
Tonometers which use gases, such as the pneumotonometer, have several disadvantages and limitations. Such device are also subject to the operator errors as with Goldman's tonometry. In addition, this device uses freon gas, which is not considered environmentally safe. Another problem with this device is that the gas is flammable and as with any other aerosol-type can, the can may explode if it gets too hot. The gas may also leak and is susceptible to changes in cold weather, thereby producing less accurate measurements. Transmission of diseases is also a problem with this type of device if probe covers are not utilized.
In conventional indentation tonometry (Schiotz), the main source of errors are related to the application of a relatively heavy tonometer (total weight at least 16.5 g) to the eye and the differences in the distensibility of the coats of the eye. Experience has shown that a heavy weight causes discomfort and raises the intraocular pressure. Moreover the test depends upon a cumbersome technique in which the examiner needs to gently place the tonometer onto the cornea without pressing the tonometer against the globe. The accuracy of conventional indentation may also be reduced by inadequate cleaning of the instrument as will be described later. The danger of transmitting infectious diseases, as with any contact tonometer, is also present with conventional indentation.
A variety of methods using a contact lens have been devised, however, such systems suffer from a number of restrictions and virtually none of these devices is being widely utilized or is accepted in the clinical setting due to their limitations and inaccurate readings. Moreover, such devices typically include instrumented contact lenses and/or cumbersome and complex contact lenses.
Several instruments in the prior art employ a contact lens placed in contact with the sclera (the white part of the eye). Such systems suffer from many disadvantages and drawbacks. The possibility of infection and inflammation is increased due to the presence of a foreign body in direct contact with a vascularized part of the eye. As a consequence, an inflammatory reaction around the device may occur, possibly impacting the accuracy of any measurement. In addition, the level of discomfort is high due to a long period of contact with a highly sensitive area of the eye. Furthermore, the device could slide and therefore lose proper alignment, and again, preventing accurate measurements to be taken. Moreover, the sclera is a thick and almost non-distensible coat of the eye which may further impair the ability to acquire accurate readings. Most of these devices utilize expensive sensors and complicated electric circuitry imbedded in the lens which are expensive, difficult to manufacture and sometimes cumbersome.
Other methods for sensing pressure using a contact lens on the cornea have been described. Some of the methods in this prior art also employ expensive and complicated electronic circuitry and/or transducers imbedded in the contact lens. In addition, some devices use piezoelectric material in the lens and the metalization of components of the lens overlying the optical axis decreases the visual acuity of patients using that type of device. Moreover, accuracy is decreased since the piezoelectric material is affected by small changes in temperature and the velocity with which the force is applied. There are also contact lens tonometers which utilize fluid in a chamber to cause the deformation of the cornea; however, such devices lack means for alignment and are less accurate, since the flexible elastic material is unstable and may bulge forward. In addition, the fluid therein has a tendency to accumulate in the lower portion of the chamber, thus failing to produce a stable flat surface which is necessary for an accurate measurement.
Another embodiment uses a coil wound about the inner surface of the contact lens and a magnet subjected to an externally created magnetic field. A membrane with a conductive coating is compressed against a contact completing a short circuit. The magnetic field forces the magnet against the eye and the force necessary to separate the magnet from the contact is considered proportional to the pressure. This device suffers from many limitations and drawbacks. For example, there is a lack of accuracy since the magnet will indent the cornea and when the magnet is pushed against the eye, the sclera and the coats of the eye distort easily to accommodate the displaced intraocular contents. This occurs because this method does not account for the ocular rigidity, which is related to the fact that the sclera of one person's eye is more easily stretched than the sclera of another. An eye with a low ocular rigidity will be measured and read as having a lower intraocular pressure than the actual eye's pressure. Conversely, an eye with a high ocular rigidity distends less easily than the average eye, resulting in a reading which is higher than the actual intraocular pressure. In addition, this design utilizes current in the lens which, in turn, is in direct contact with the body. Such contact is undesirable. Unnecessary cost and complexity of the design with circuits imbedded in the lens and a lack of an alignment system are also major drawbacks with this design.
Another disclosed contact lens arrangement utilizes a resonant circuit formed from a single coil and a single capacitor and a magnet which is movable relative to the resonant circuit. A further design from the same disclosure involves a transducer comprised of a pressure sensitive transistor and complex circuits in the lens which constitute the operating circuit for the transistor. All three of the disclosed embodiments are considered impractical and even unsafe for placement on a person's eye. Moreover, these contact lens tonometers are unnecessarily expensive, complex, cumbersome to use and may potentially damage the eye. In addition none of these devices permits measurement of the applanated area, and thus are generally not very practical.
The prior art also fails to provide a sufficiently accurate technique or apparatus for measuring outflow facility. Conventional techniques and devices for measuring outflow facility are limited in practice and are more likely to produce erroneous results because both are subject to operator, patient and instrument errors.
With regard to operator errors, the conventional test for outflow facility requires a long period of time during which there can be no tilting of the tonometer. The operator therefore must position and keep the weight on the cornea without moving the weight and without pressing the globe.
With regard to patient errors, if during the test the patient blinks, squeezes, moves, holds his breath, or does not maintain fixation, the test results will not be accurate. Since conventional tonography takes about four minutes to complete and generally requires placement of a relatively heavy tonometer against the eye, the chances of patients becoming anxious and therefore reacting to the mechanical weight placed on their eyes is increased.
With regard to instrument errors, after each use, the tonometer plunger and foot plate should be rinsed with water followed by alcohol and then wiped dry with lint-free material. If any foreign material drys within the foot plate, it can detrimentally affect movement of the plunger and can produce an incorrect reading.
The conventional techniques therefore are very difficult to perform and demand trained and specialized personnel. The pneumotonograph, besides having the problems associated with the pneumotonometer itself, was considered "totally unsuited to tonography." (Report by the Committee on Standardization of Tonometers of the American Academy of Ophthalmology; Archives Ophthalmol., 97:547-552, 1979). Another type of tonometer (Non Contact "Air Puff" Tonometer-U.S. Pat. No. 3,545,260) was also considered unsuitable for tonography. (Ophthalmic & Physiological Optics, 9(2):212-214, 1989). Presently there are no truly acceptable means for self-measurement of intraocular pressure and outflow facility.
In relation to an additional embodiment of the present invention, blood is responsible not only for the transport of oxygen, nutrients, glucose, cholesterol, electrolytes, water, enzymes, white and red blood cells, and genetic markers, but also provides an enormous amount of information in regards to the overall health status of an individual. The prior art related to analysis of blood relies primarily on invasive methods such as with the use of needles to draw blood for further analysis and processing. Very few and extremely limited methods for non-invasive evaluating blood components are available.
In the prior art for example, oxygenated hemoglobin has been indirectly measured by a pulse oximeter based on traditional near infrared absorption spectroscopy and indirectly measures arterial blood oxygen carried by hemoglobin (not molecular concentration of oxygen) with sensors placed over the skin utilizing LEDs emitting at two wave lengths around 940 and 660 nanometers. As the blood oxygenation changes, the ratio of the light transmitted by the two frequencies changes indicating the amount of oxygenated hemoglobin in the arterial blood of the finger tip. The present systems are not accurate and provide only the amount of oxygenated hemoglobin in the finger tip.